The invention relates to the field of acoustic measurement and calibration and, more particularly to a standard and device useful in testing and calibrating ultrasonic equipment and generating predictable and reproducible sounds.
A convenient way to test the performance of acoustic equipment is to compare its acoustic emission response to an acoustic source emitting uniformly repeatable sound waves. The comparison is dependent on the acoustic source being able to reproduce sounds reliably and accurately. The acoustic source then serves as an acoustic measurement or calibration standard against which comparisons are made. Repeatable acoustic sounds are generated according to a method known in the art, by carefully breaking a Hsu pencil lead against a test block. The use of a Hsu pencil in generating repeatable sound waves is described in ASTM E976-94, xe2x80x9cStandard Guide for Determining the Reproducibility of Acoustic Emission Sensor Response,xe2x80x9d pp. 388-390, which is incorporated by reference herein. When the lead of the Hsu pencil breaks, there is a sudden release of the stress on the surface of the test block where the lead is touching. The stress release generates an acoustic wave. In generating reproducible, uniform sound waves, care is taken to always break the same length of the same type of lead, and to break the lead at the same spot on the test block with the same angle and orientation of the Hsu pencil.
There are disadvantages inherent in the conventional method using a Hsu pencil for generating repeatable sounds. Great care and effort must be expended to ensure uniformity in the breaking of the Hsu pencil in order to reliably produce repeatable sounds. Additional devices, such as a Nielsen shoe (as shown in ASTM E976-94xe2x80x9cStandard Guide for Determining the Reproducibility of Acoustic Emission Sensor Responsexe2x80x9d, p. 390), are required to achieve that uniformity. Furthermore, once a Hsu pencil is broken to produce a sound, it is no longer reusable.
Calibration standards, and devices therefor, are useful in calibrating ultrasonic instruments used in locating and estimating measurements of gas leakage. Exemplary methods in the art for measuring gas leakage using airborne ultrasonic techniques are described in ASTM E 1002-96, xe2x80x9cStandard Test Method for Leaks Using Ultrasonics,xe2x80x9d pp. 422-424, which is also incorporated by reference herein.
FIG. 1 is a block diagram illustrating a prior art ultrasonic airborne calibration standard for gas leakage. Airborne ultrasound is a pressure wave or longitudinal wave that travels through air. It is produced by either turbulent flow or a vibrating surface. Referring to FIG. 1, there is shown a nitrogen gas supply 11 and a regulator 13 for regulating the gas flow from the nitrogen gas supply 11. The nitrogen gas supply 11 is a pressurized gas source that creates the equivalent of a nitrogen gas leak through an orifice 15. A sound absorbing barrier 17 is selectively placed in front of the orifice 15 to intercept the pressure wave and stop it or removed to allow a uniform wave to be received by air probe 19. Since the wave is uniform because of the regulator, the apparatus forms a standard for gas leakage, which can be used to calibrate probe 19.
FIG. 2 is a flow diagram illustrating the operation of a prior art ultrasonic calibration method for measuring gas leakage using the equipment shown in FIG. 1. Referring to FIG. 2 (in conjunction with FIG. 1), the regulator 13 regulates the gas flow from nitrogen gas supply 11 to a leak standard, e.g., 4.9xc3x9710xe2x88x925 mol/s(1.1 std. cm3/s at 0xc2x0 C.) xc2x15% (step 21). The size of the orifice 15 of the nitrogen gas supply 11 is approximately 0.2 mm (0.008 inches). Since the calibration is conducted in the airborne mode for ultrasound received in a gaseous medium, the air probe 19 is positioned at a distance D1 of 10 meters (xc2x10.1 m) from the orifice 15 (step 23). In step 25, the air probe 19 and the orifice 15 are aligned to obtain peak acoustic response. The flow rate of the gas emitted from the nitrogen gas supply 11 is scaled back 50% of the full scale (xc2x15%). The sound absorbing barrier 17 is placed in front of the orifice 15 blocking out the calibrated gas leak (step 27). The meter reading of the flow rate of the gas being emitted from the nitrogen gas supply 11 is checked to see if it is equal to zero, that is, no audible signal detected (step 29). This calibration process is repeated for each level used in detecting and measuring gas leakage (step 28).
Application of the methodology described in FIGS. 1 and 2 has become impractical today since many industrial environments do not have the space for such a great testing distance (e.g., D1=10 m) with no competing ultrasound. Further, the long testing distance results in low acoustic sensitivity, which has a negative impact on the accuracy of the ultrasonic calibration. Also, in general calibration devices in the art can perform ultrasonic calibration in the airborne mode only, but not in the structure-borne mode for ultrasound received through a solid medium.
A reliable, easy-to-use methodology and device therefor are thus needed for ultrasonic calibration which overcome the problems in the prior art. There is a further need in the art for an ultrasonic calibration method and device for both the airborne mode and the structure-borne mode.
The present invention relates to a device that generates predictable and reproducible sounds. The device according to the invention serves as a standard to calibrate ultrasonic measuring equipment through either a gaseous medium or a solid medium.
In an exemplary embodiment, the acoustic calibration device according to the invention is similar in basic shape to an hourglass or venturi (e.g., a sand-type egg-timer), but instead of using fine particles, such as sand, the device includes uniform media particles (e.g., marbles, balls, beads, etc.) which drop from an upper chamber to a lower one. The upper chamber is connected to the lower chamber, with a neck disposed therebetween. The media particles are pre-sorted so that those sealed within the hourglass structure are of uniform size. An impingement post (e.g., a bell or sounding plate) is positioned in the path of the falling media particles so that each will hit the sounding plate, resulting in a reverberating sound. The neck allows the passage of only a controlled number of the media particles per unit of time. The impingement post is cantilevered and supported through a connection to the hour glass structure.
The device according to the invention is advantageous over the prior art because it is easy to use and carry, and reliably provides a uniform acoustic output of repeatable sounds without the need for an external energy source. The device according to the invention does not require a large testing space nor additional devices aiding the ultrasonic calibration process. The invention is further advantageous over the prior art in that the device according to the invention can be used for ultrasonic calibration in both the airborne mode and the structure-borne mode.